SIM troubles

I have had my 9700 for over 3 years, but from july to now I've been using a 9900 to hold me over until bb10 but now I am selling it to fund my new device, so Im back to using my 9700. The problem is, after not using the phone for an hour or 2, like when I sleep for example, it says invalid SIM and I need to reboot it. Also, in phone calls it constantly drops.

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